ASVAB — All Questions

According to Ohm's law, voltage equals current multiplied by:

• a.power
• b.resistance✓
• c.frequency
• d.capacitance

Ohm's law states that voltage equals current times resistance, written as V equals I times R. This relationship links the three basic quantities in a circuit.

The unit used to measure electrical resistance is the:

• a.watt
• b.ohm✓
• c.volt
• d.ampere

Resistance is measured in ohms, symbolized by the Greek letter omega. The volt measures voltage, the ampere measures current, and the watt measures power.

A device that stores electrical charge is a:

• a.inductor
• b.resistor
• c.transformer
• d.capacitor✓

A capacitor stores electrical energy in an electric field between two plates. A resistor limits current, while an inductor stores energy in a magnetic field.

In a circuit, current is measured in:

• a.farads
• b.ohms
• c.amperes✓
• d.volts

Electric current, the flow of charge, is measured in amperes, often shortened to amps. Ohms measure resistance, volts measure voltage, and farads measure capacitance.

What is the total resistance of two 10-ohm resistors connected in series?

• a.10 ohms
• b.5 ohms
• c.100 ohms
• d.20 ohms✓

In a series circuit, resistances add together directly. Ten ohms plus ten ohms equals 20 ohms. In parallel, the total would instead be lower than either resistor.

Electrical power in watts is calculated by multiplying voltage by:

• a.current✓
• b.resistance
• c.capacitance
• d.time

Power equals voltage times current, written as P equals V times I. This gives the rate at which electrical energy is used, measured in watts.

A component that allows current to flow in only one direction is a:

• a.diode✓
• b.switch
• c.resistor
• d.capacitor

A diode conducts current in one direction and blocks it in the other, acting like a one-way valve. This property makes diodes useful for converting AC to DC.

The abbreviation AC stands for:

• a.alternating current✓
• b.active charge
• c.ampere current
• d.automatic circuit

AC stands for alternating current, in which the direction of electron flow reverses periodically. DC, or direct current, flows steadily in one direction.

Which material is the best conductor of electricity?

• a.Copper✓
• b.Glass
• c.Rubber
• d.Wood

Copper is an excellent conductor and is widely used in electrical wiring. Rubber, glass, and wood are insulators that resist the flow of current.

A transformer is used to change the level of:

• a.resistance
• b.current direction
• c.voltage✓
• d.frequency

A transformer increases or decreases AC voltage using two coils of wire. Stepping voltage up or down helps electricity travel efficiently and power different devices.

If voltage is 12 volts and resistance is 4 ohms, what is the current?

• a.3 amperes✓
• b.48 amperes
• c.4 amperes
• d.2 amperes

Using Ohm's law, current equals voltage divided by resistance. That is 12 volts divided by 4 ohms, which equals 3 amperes.

A material that strongly resists the flow of electric current is called a(n):

• a.semiconductor
• b.battery
• c.conductor
• d.insulator✓

An insulator resists the flow of electricity and is used to protect and separate conductors. Rubber and plastic are common insulators used to coat wires.

The component that provides a source of stored electrical energy in a portable device is the:

• a.battery✓
• b.fuse
• c.switch
• d.antenna

A battery stores chemical energy and converts it to electrical energy to power a device. A fuse protects the circuit, and a switch simply opens or closes it.

A fuse protects a circuit by:

• a.increasing the voltage
• b.breaking the circuit when current is too high✓
• c.reversing the current
• d.storing extra charge

A fuse contains a thin wire that melts and breaks the circuit when current exceeds a safe level. This prevents overheating and damage to the rest of the circuit.

A 12-volt source is connected across a 4-ohm resistor. How much current flows?

• a.48 amperes
• b.3 amperes✓
• c.8 amperes
• d.0.33 ampere

Ohm's law gives current as voltage divided by resistance: 12 / 4 = 3 amperes. Multiplying instead of dividing would give the 48 in one of the wrong choices.

Electrical resistance is measured in which unit?

• a.ohms✓
• b.watts
• c.volts
• d.amperes

Resistance is measured in ohms. Volts measure potential difference, amperes measure current, and watts measure power.

Three resistors are wired in series. How does the current through the second resistor compare with the current through the first?

• a.it is one-third as large
• b.it is the same✓
• c.it is three times as large
• d.it is twice as large

A series circuit offers only one path, so the same current passes through every element. It is the voltage that divides among series resistors.

What is the total resistance of two 6-ohm resistors connected in parallel?

• a.12 ohms
• b.3 ohms✓
• c.36 ohms
• d.6 ohms

Two equal resistors in parallel give half the resistance of one, so 6 / 2 = 3 ohms. Parallel resistance is always less than the smallest branch.

What is the total resistance of a 4-ohm and an 8-ohm resistor connected in series?

• a.4 ohms
• b.32 ohms
• c.2.67 ohms
• d.12 ohms✓

Series resistances add directly: 4 + 8 = 12 ohms. The 2.67 figure would be the result if they were in parallel.

A device draws 5 amperes from a 120-volt supply. How much power does it use?

• a.24 watts
• b.1,200 watts
• c.600 watts✓
• d.125 watts

Power equals voltage times current: 120 x 5 = 600 watts.

Which of these materials is the best conductor of electricity?

• a.rubber
• b.copper✓
• c.dry wood
• d.window glass

Copper has many free electrons and is used for most wiring. Rubber, wood, and glass are insulators.

How does a fuse protect a circuit?

• a.a metal strip melts and opens the circuit✓
• b.a coil stores the excess current safely
• c.a switch closes to divert the current
• d.a resistor absorbs the extra voltage

Excess current heats the fuse element until it melts, breaking the circuit. A fuse can only open a circuit; it cannot store or absorb current.

What is the main function of a transformer?

• a.to measure circuit current
• b.to convert AC into DC
• c.to store electrical charge
• d.to raise or lower AC voltage✓

A transformer couples two coils magnetically to change voltage. It needs a changing current, so it does not work on steady DC, and converting AC to DC is the job of a rectifier.

What does a diode do in a circuit?

• a.it increases the voltage across a load
• b.it converts current into magnetism
• c.it stores charge between its two plates
• d.it passes current in one direction only✓

A diode conducts when forward biased and blocks when reverse biased, which is why diodes are used to rectify alternating current.

A capacitor stores energy in

• a.a mechanical spring
• b.a magnetic field
• c.a chemical reaction
• d.an electric field✓

Charge collects on the capacitor's plates and the energy resides in the electric field between them. An inductor stores energy in a magnetic field instead.

What is the frequency of standard household alternating current in the United States?

• a.50 hertz
• b.120 hertz
• c.60 hertz✓
• d.110 hertz

United States utilities supply 60 hertz. Much of Europe and Asia uses 50 hertz instead.

What is the nominal voltage of a standard household outlet in the United States?

• a.12 volts
• b.120 volts✓
• c.480 volts
• d.24 volts

Ordinary branch circuits supply about 120 volts. Large appliances such as electric ranges use a 240-volt circuit.

In United States residential wiring, which insulation color identifies the equipment grounding conductor?

• a.black
• b.green✓
• c.red
• d.white

Grounding conductors are green, green with a yellow stripe, or bare copper. White marks the grounded neutral, and black and red mark ungrounded hot conductors.

Four lamps are wired in parallel across a supply. If one lamp burns out, what happens to the others?

• a.they all go dark
• b.they keep burning✓
• c.they burn much brighter
• d.they burn much dimmer

Each branch of a parallel circuit has its own path to the source, so an open branch does not interrupt the others. In a series string, one failure would darken them all.

Which change would increase the resistance of a length of copper wire?

• a.making the wire thicker
• b.making the wire longer✓
• c.cooling the wire down
• d.coiling the wire loosely

Resistance rises with length and falls as cross-sectional area increases. Cooling a metal conductor also lowers its resistance.

How is an ammeter connected in order to read the current in a circuit?

• a.across the supply terminals
• b.in parallel with the load
• c.between the load and ground
• d.in series with the load✓

The current to be measured has to pass through the ammeter, so it goes in series. An ammeter placed in parallel presents a near short circuit.

How is a voltmeter connected in order to read the voltage across a component?

• a.in parallel with the component✓
• b.in series with the component
• c.in series with the supply
• d.between the two supply fuses

Voltage is a difference between two points, so the meter bridges those two points in parallel. A voltmeter has very high resistance so it draws almost no current.

Which material is the basis of most modern semiconductor devices?

• a.aluminum
• b.silicon✓
• c.rubber
• d.copper

Silicon conducts far better than an insulator and far worse than a metal, and doping it controls that conduction precisely.

A circuit carries 2 amperes when 24 volts is applied. What is its resistance?

• a.12 ohms✓
• b.2 ohms
• c.48 ohms
• d.26 ohms

Resistance equals voltage divided by current: 24 / 2 = 12 ohms.

What is the main purpose of a resistor in a circuit?

• a.to boost voltage
• b.to rectify current
• c.to store charge
• d.to limit current✓

A resistor opposes current flow, which sets the current in a branch and divides voltage. Storing charge is a capacitor's job.

Electrical power is measured in which unit?

• a.ohms
• b.amperes
• c.watts✓
• d.coulombs

One watt is one joule per second. Ohms measure resistance, coulombs measure charge, and amperes measure current.

A step-down transformer has which winding arrangement?

• a.equal turns on both windings
• b.more turns on the secondary
• c.no winding on the secondary
• d.fewer turns on the secondary✓

Output voltage follows the turns ratio, so fewer secondary turns give a lower output voltage and a proportionally higher output current.

Two 1.5-volt cells are connected in series. What is the voltage across the pair?

• a.0.75 volt
• b.3.0 volts✓
• c.4.5 volts
• d.1.5 volts

Cells in series add their voltages: 1.5 + 1.5 = 3.0 volts. Connecting them in parallel would keep the voltage at 1.5 while doubling the available current.